From mud to mol

Representing the section “Marine Geochemistry” of the Alfred Wegener Institute Bremerhaven and in collaboration with colleagues from the Jacobs University in Bremen, Jennifer Ciomber, Jessica Volz, Vincent Ozegowski and I are here onboard RV SONNE to investigate the possible impacts of deep-sea mining in the Clarion Clipperton Fracture Zone from a geoscientific perspective. More specifically, we would like to observe how the biogeochemical environment at and in the seafloor responds to disturbance events, that is whether it re-equilibrates and if so, how long it takes.

During the current EcoResponse cruise we have the opportunity to investigate tracks and traces of past disturbance events, such as those from 30-year old mining tests. Additionally, we are able to conduct new controlled disturbance experiments. This allows us to examine responses over different time scales (from years to decades) and also to detect changes immediately after the disturbance.

In order to achieve our goals we process sediment collected by the gravity corer, the multi-corer and push cores retrieved by the ROV Kiel 6000. The gravity corer (Picture 1) is essentially a long metal pipe, which drills itself into the sediment with the force of its own weight. By using this simple but very effective gear we can dig up to 10 m into the seafloor. However, as the gravity corer weighs several tons it tends to disturb the uppermost part of the sediment. Therefore another sampling device – the multi-corer – is needed. It can only retrieve the upper 30-40 cm of the sediment, but delivers an undisturbed sediment-water interface. In addition to these conventional sampling devices small push cores of about 20 cm length are collected by the ROV. These cores may be very short, though the ROV can place the push cores with high precision and thus enables us to sample 1 to 2 meter wide tracks in water depths of more than 4000 m.

Getting the samples back to the vessel and into the labs is part of our work as geoscientists. This is only the first step, though. The obtained sediment has to be sampled for its solid as well as its fluid constituents. We use rhizons, in combination with syringes (Picture 2) to extract the interstitial water from the sediment. This work has to be done in the cold lab at around 4°C reflecting in-situ (deep-sea) conditions to preserve the samples. From on-board analyses of the interstitial water (Picture 3) we quickly received first interesting results, but further analyses will need to be carried out in the labs back home. For this purpose, great care is taken to avoid exposure of our samples to the atmosphere and, thus, to ensure the quality of future analysis.

After retrieving the sediment from the deep sea as well as sampling and analyzing procedures we finally end up with concentration profiles across sediment depth for numerous elements and compounds. All the element cycles are interacting with each other in a complex system of precipitation, dissolution, transport and reaction processes. Understanding this system also requires consideration of environmental factors such as sedimentation rate, seafloor morphology and many more. Biogeochemical processes play a significant role both at a regional and global scale, such as the Earth’s climate or the distribution and abundance of manganese nodules. It is therefore necessary to enhance our understanding of the biogeochemical processes in the seafloor. Thus, with our research undertaken during the EcoResponse cruise onboard RV SONNE we are hoping to gain some new and valuable information for the Clarion Clipperton Fracture Zone.